This proposal aims to contribute to an understanding of the organization of elementary genetic units in higher organisms, and the regulation of the expression of such units in development. The rosy locus of chromosome 3 in Drosophila melanogaster carries the structural information for a peptide which, as a homodimer, functions as xanthine dehydrogenase in this organism. Mapping experiments have served to map the borders of the structural element and variants which map outside and just to the left of the structural element appear to be associated with regulation (i.e., they are alterations in a cis-acting control element). The direction of experiments in the current proposal involve a continuation of previous work, and involves: (1) Mutagenesis experiments involving selective strategies designed to recover several classes of control variants, (2) analysis of such variants involving immunochemical, thermal, kinetic, cytogenetic, fine structure recombination and developmental studies designed to document these variants, (3) two of our previously recovered apparent control element variants are really rearrangements that place centromeric heterochromatin adjacent to the rosy locus and are classic position-effect variants. Experiments during the coming year are designed to provide further molecular characterization of the position-effect phenomenon. (4) Collaborative experiments with Dr. Welcome Bender of Harvard University, making use of recombinant DNA technology, are designed to pursue relevant questions at the molecular level.